1. Field of the Invention
This invention generally relates to a method and apparatus for real-time, in situ measurement of fuel gas compositions and heating values. In one aspect, this invention relates to the use of infrared sensors for measuring fuel gas compositions. In one aspect, this invention relates to the use of semiconductor sensors for measuring fuel gas compositions. In one aspect, this invention relates to a method for determining the heating value of fuel gas compositions.
2. Description of Related Art
Globally increasing demand for energy and volatility in supply and pricing of natural gas and fossil fuels along with increasingly more stringent environmental restrictions, such as calls for reducing carbon emissions, have lead to growing interest in the use of alternative fuels or fuel gases from other sources such as landfill gas and producer gas, including, for example, syngas, coke oven gas, refinery gas and coalbed gas. Fuel-flexible engines, turbines, burners, and the like are being developed to better permit the use of these alternative fuels and their blends with natural gas. The reliable and efficient use of such fuel gases in engines and turbines requires proper design and operation such as to properly maintain combustion, stability, emission levels, output and efficiency. Natural gas and alternative fuels can, however, present wide variation in compositions and heating values, such as dependent on their source and treatment to which they may be subjected, whereas engines and other combustion equipment are typically designed to operate only within a specific range of fuel compositions and energy content. Thus, in order to avoid or prevent shutdowns and/or damage to such engines and equipment as well as to improve process efficiency, it is highly desirable to be able to monitor the composition of the incoming fuel and to adapt the air-fuel ratio accordingly. Further, as combustion is a very fast process, the analysis and measurement of fuel compositions and heating values must necessarily be correspondingly fast as well.
At present, gas chromatography is the most widely or commonly used method for fuel gas composition analysis and measurement. Gas chromatography, however, typically requires at least several minutes to analyze a gas sample and, thus, does not essentially provide real-time information of fuel gas properties. Calorimeters, which are used to measure the energy content of a fuel gas, have cost and response times that are similar to gas chromatographs and they only can measure the energy content of the fuel gas and not the fuel gas composition.
As used herein, the term “producer gas” refers to gas mixtures containing primarily hydrocarbons, carbon monoxide (CO), carbon dioxide (CO2), hydrogen (H2), and nitrogen (N2). However, not all of these gases can be detected using a single inexpensive sensor. One solution to this problem is to employ a number of different sensors, each of which is intended for the detection of one or more of these gases. Known sensors suitable for use in detecting these gases are, however, problematic due to the fact that such gas sensors are generally cross-sensitive to one or more gases other than the specific target gas of the sensor. For example, as shown in FIGS. 1 and 2, palladium-based hydrogen sensors are highly cross-sensitive to methane and optical carbon monoxide sensors are cross-sensitive to ethane and butane. Thus, the use of such sensors to measure the fuel gas composition and heating value of fuel gas mixtures in real time requires proper compensation for the affects of these cross-sensitivities on the measured values produced by these sensors.
In the past few years, due to the advent of fast computing technology, multivariate regression methods, mainly, principal component regression (PCR) and partial least squares (PLS), have emerged as a promising tool for many analytical techniques. The use of near infrared (NIR) absorption spectroscopy and multivariate regression for measuring the composition and heating value of natural gas mixtures and characterizing landfill gas and synthesis gas (syngas) is known. Raman scattering can also be used to detect and measure virtually all of the components of fuel gas mixtures such as natural gas and biogas. It is also known that other physical properties of a variety of fuels ranging from gasoline and jet to diesel can be accurately predicted using multivariate modeling of NIR, FTIR (Fourier transform infrared spectroscopy), and FT-Raman measurements.
NIR sensors are significantly less expensive than Raman-based sensors. However, not all the components of conventional and alternative fuels, e.g., hydrogen, absorb light in the NIR range.